The present invention relates generally to mounting films on substrates, and more particularly, to stretching and mounting fragile films on substrates.
Flexible films are used for conducting electrical charges, radiating energy, reflecting incident energy, protecting and beautifying surfaces, and for insulating electric-current carrying wires and sheets, as typical examples. Thin films comprised of living tissue are becoming important for certain surgical procedures. Currently, electronic circuits are being manufactured on thin, flexible film materials, since they can then be bent, folded, and twisted, thereby providing design possibilities otherwise impossible to achieve. Furthermore, in many applications, film materials are more uniform and otherwise superior to surface coatings applied by spraying, dipping, or brushing. However, applications of thin or fragile films has been limited since handling such films often introduces wrinkles into the film.
Several methods for mounting films on substrates have been described. Generally, these methods employ thermal bonding, and heat and pressure are applied to the film/backing combination in order to remove trapped air and provide a uniform bond. For example, in "Method Of Securing Flexible Sheets To Relatively Stiff Backings," by Donald J. Douglas, U.S. Pat. No. 2,620,289, which issued on Dec. 2, 1952, pressure is applied to the outer surface of a film which has been coated with an adhesive and placed in contact with a backing material, in order to cause it to conform to the backing, while air is withdrawn from between the surfaces, by placing the assembly in a vacuum chamber. Heat is then applied to activate and set the adhesive. "Method And Apparatus For Making An Optical Element Having A Dielectric Film," U.S. Pat. No. 4,684,424, which issued to Gordon C. Augason on Aug. 4, 1987, describes an apparatus for sandwiching a film between "O" rings held in "O" ring bearing members and an optical element to be coated, on each side thereof, and stretching the films across the optical element by squeezing the bearing members together. Tapered surfaces in the "O" ring bearing cause the "O" rings to roll outwardly when the members are squeezed together, thereby stretching the film. The film to be used as a coating is further sandwiched between a Mylar film and the optical element, and air is withdrawn from between the optical element and the coating film in order to assist in removing entrained air and wrinkles. The Mylar is utilized both to press the coating film against the element on both sides thereof and to hold the element in place while the coating film is heated to slightly below its melting temperature so that thermal bonding may take place. The apparatus may be used to stretch a single film as well. In order to achieve the stretching of a film, wing nuts are carefully tightened causing the "O" rings to roll outwardly, while the film is visually observed. A similar process is used for coating optical elements. It should be noted that the coating film is not bonded to the Mylar film. Yet another film stretching procedure is described in U.S. Pat. No. 4,463,055 for "Reflective Film And Method Of Applying Same," which issued on Jul. 31, 1984 to Marvin P. Hodges. Therein, the inventor affixes a thin reflective foil to a polymeric carrier, which is then fitted to a complex surface without rupturing or cracking the foil. The method requires that the foil be laminated to the carrier; that is an adhesive is used to bond the two surfaces together at all points of contact therebetween.
In U.S. Pat. No. 5,125,994, "Thermoforming Method," which issued to Louis P. Harasta et al. on Jun. 30, 1992, the inventors describe the stretching and bonding of an adhesive-coated thermoplastic film to a substrate by vacuum thermoforming. The method combines heating a film, which is rigidly held at its outer dimensions, and reducing the pressure behind it in order to cause the film to cover the surface of the substrate and bond thereto with a minimum of air entrapment. The combination of heating and applying vacuum to the film causes the film to be stretched.
Accordingly, it is an object of the present invention to provide an apparatus for stretching films utilizing vacuum to generate the tensioning force.
It is another object of the present invention to provide an apparatus for further stretching a film already stretched utilizing vacuum to generate the tensioning force, by utilizing gas pressure to expand the film.
It is yet another object of the present invention to provide a method for stretching films by utilizing a backing film which is stretched.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.